1. Field of the Invention
The present invention concerns a process of oven brazing in rarefied or controlled atmosphere of two pieces having very different coefficients of expansion and/or destined to be exposed to high thermic gradients.
2. Description of the Prior Art
In general, we know that in the classical oven brazing technique the pieces to be brazed are first assembled, for example with the aid of annex elements, so that the zones of contact where the brazing is to take place are pressed up against one another in their definitive positions, with the interposition of the contributing metallic compound (brazing metal) which is usually in the form of a thin plaque, brazing wire or paste, depending on the conception of the pieces to be brazed.
This assembled whole is then placed in a thermic treatment oven capable of ensuring the appropriate thermic cycle, during which the two pieces are heated to a high temperature bringing about the fusion of the contributing metallic compound which, in liquid state, can interpenetrate to a certain depth the matter constituting the two pieces around the contact zones. At the end of the cycle, the pieces undergo a cooling phase which brings about the solidification of the contributing metallic compound and the return of the whole to the ambient temperature.
To allow for the nature of the pieces to be brazed and the contributing metallic compound used, this thermic cycle can be carried out in controlled atmosphere or even in vacuum conditions, notably in view of avoiding untimely chemical reactions and, in particular, oxidisation phenomena that might jeopardize the quality of the brazing, this solution having moreover the advantage of doing without the use of a flux.
This technique is quite suitable where the two pieces to be brazed have similar coefficients of expansion. On the other hand, in the case of the two pieces having very different coefficients of expansion, differential expansion phenomena occurring around the junction zone of the two pieces may lead, in particular during the cooling phase, to detachment of the brazing, as well as deteriorations, and even to ruptures of one and/or the other of the two pieces.
Of course, these faults may also become manifest in the case of the two pieces, once brazed, being exposed to high rises in temperature.
Similarly, these phenomena of differential expansion can come into play in the case of two pieces having similar coefficients of expansion being exposed, subsequent to brazing, to different temperatures.
This is notably the case of certain elements used in thermic exchangers that are made of two pieces joined together by brazing, one piece being heated to high temperature while the other is constantly cooled.